The worldwide increase in energy demands, the constant rise in energy prices and concerns of environmental damages caused by the increased use of fossil energy resources drives the demand for energy conservation and the use of renewable energies. The problem can be solved by the implication of textile heat accumulators based on the application of phase change material and used for hot water supply.
Phase change material is a highly-productive thermal storage medium which possesses the ability to change its physical state within a certain temperature range. When the melting temperature is obtained during a heating process, the phase change from the solid to the liquid state occurs. During this melting process, the phase change material absorbs and stores a large amount of latent heat. The temperature of the phase change material remains nearly constant during the entire process. When the phase change is complete, a continuing heating process leads to a further temperature increase and the absorption of a much smaller amount of sensible heat. In a cooling process of the phase change material, the stored latent heat is released into the environment in a certain temperature range, and a reverse phase change from the liquid to the solid state takes place. During this crystallization process, the temperature of the phase change material also remains constant. The high heat transfer during the melting process and the crystallization process, both without any temperature change, is responsible for the phase change material's appeal as a source of heat storage.
In order to contrast the amount of latent heat absorbed by a phase change material during the actual phase change with the amount of sensible heat absorbed in an ordinary heating process, the ice-water phase change process will be used. When ice melts, it absorbs an amount of latent heat of about 335 J/g. When the water is further heated, it absorbs a sensible heat of only 4 J/g while its temperature rises by one degree C. Thus, water needs to be heated as long as its temperature rises from 1° C. to about 84° C. in order to absorb the same amount of heat which is absorbed during the melting process of ice.
In addition to ice (water), more than 500 natural and synthetic phase change materials are known. These materials differ from one another in their phase change temperature ranges and their latent heat storage capacities.
Currently, crystalline alkyl hydrocarbon phase change materials having different chain lengths are used in textile applications and more specifically in garment applications. Characteristics of these phase change materials are summarized in Table 1.
TABLE 1Crystalline alkyl hydrocarbonsLatent heatCrystallineMeltingCrystallizationstoragealkyltemperature,temperature,capacity,hydrocarbonsFormula° C.° C.J/gHeneicosaneC21H4440.535.9213EicosaneC20H4236.130.6247NonadecaneC19H4032.126.4222OctadecaneC18H3828.225.4244HeptadecaneC17H3621.716.5213HexadecaneC16H3416.712.2237
The crystalline alkyl hydrocarbons are either used in technical grades with a purity of approximately 95%; or they are blended with one another in order to cover specific phase change temperature ranges. The crystalline alkyl hydrocarbons are nontoxic, non-corrosive, and non-hygroscopic. The thermal behavior of these phase change materials remains stable under permanent use. Crystalline alkyl hydrocarbons are byproducts of petroleum refining and, therefore, inexpensive. A disadvantage of crystalline alkyl hydrocarbons is their low resistance against ignition.
Salt hydrates are alloys of inorganic salts and water. The most attractive properties of salt hydrates are the comparatively high latent heat storage capacities, the high thermal conductivities and the small volume change during melting. They are mostly non-combustible which makes them specifically attractive for building applications. Salt hydrates often show an incongruent melting behaviour as a result of a lack in reversible melting and freezing making them unsuitable for permanent use. Salt hydrates with reversible melting and freezing characteristics are summarized in Table 2.
TABLE 2Salt hydratesLatentMeltingheat storagetemperature,capacity,Salt hydrates° C.J/gCalcium cloride hexahydrate29.4170Lithium nitrate trihydrate29.9296Sodium hydrogen phosphate dodecahydrate36.0280Sodium thiosulfate pentahydrate49.0200Lithium acetate dihydrate56.0270Magnesium cloride tetrahydrate58.0180
Phase change materials have been suggested for the use in solar energy systems. For instance, U.S. Pat. No. 5,269,851 describes a solar energy system where phase change material is used to protect photovoltaic cells from excessive temperatures.
U.S. Pat. No. 5,505,788 reports a photovoltaic roofing assembly where phase change material is used for temperature regulation.
Furthermore, phase change material is used for intermediate heat storage in a water heating unit described in U.S. Pat. No. 6,047,106.